Underpowered

[Stee6043]

Well put. The difference from the 340/40 at 3500rpm is the mph. But the fuel burn will be more overall because it takes much more to get them on plane also. 36gph I’m at 31mph in my boat. The 40 will cruise at 23-24 at 3500-3600. I still feel the 8.1 is really working all the time in that 40. Just my opinion.

37.6gph and you're not running from me too terribly quickly:

 

[dtfeld]

Well put. The difference from the 340/40 at 3500rpm is the mph. But the fuel burn will be more overall because it takes much more to get them on plane also. 36gph I’m at 31mph in my boat. The 40 will cruise at 23-24 at 3500-3600. I still feel the 8.1 is really working all the time in that 40. Just my opinion.

Engines are “working” or putting out the same amount of power for a given RPM, just going to have to do it longer to get where you’re going in the bigger boat.

Pushing up the RPMs to get the same cruise speed in 40’ as a 34’…now that’s making them work harder!

 

[mnm99]

37.6gph and you're not running from me too terribly quickly:

Wow. 7.4's, 3800ish and only 37.6gph. Impressive.

 

[K-DUB]

Also keep in mind the larger boat has a lot more hull planing area, and the mechanicals are further forward so they are not as tail heavy as the 340. They actually plane faster, run more level and stay in plane longer at slower speeds, regardless of number of passengers or gear. That eases the load on the engines, so it’s not an apples to apples comparison.
I run around 25mph, at 3300 and have never had a moment I felt I needed more. It’s thirsty, but only boating for four months out of the year you’ll never make up the cost in fuel savings. I say just do it!

 

[dcwjd]

Here I am. I bought the boat not wanting or needing a speed demon. The 390 is comfortable boat. Good looks and like @Great Lakes indicated mine uses about 32 gallons per hour at 30 mph. The boat is happy at 3200 rpm.

There was a full right up on the 390 in one of the magazines. They tested the boat with the gassers and were happy with the performance. There was also a diesel being tested at the same time and they did an informal drag race. They said it was surprisingly a dead heat.

If the boat checks out, I wouldn't be afraid of the engines.

This cannot be the correct information. There is no way a gas powered 39/40 is cruising at 30mph at 3200 rpm’s. A 340 with the same motors doesn’t even do that.

 

[Jus Cruisin]

This cannot be the correct information. There is no way a gas powered 39/40 is cruising at 30mph at 3200 rpm’s. A 340 with the same motors doesn’t even do that.

Those were meant to be 2 different observations. I'm not running 30 mph at 3200 rpm. I said the boat is "happy" at 3200 rpm which is 20 something.

 

[mnm99]

I get the best Mpg at 3450rpm/ 29mph, Standard 8.1's. My friends HO's like 3500-3600. Any lower they feel like they want a little more.

 

[Stee6043]

Wow. 7.4's, 3800ish and only 37.6gph. Impressive.

Full disclosure...I think I was on the bottom half of my gas tanks at that point. I pick up 1-1.5mph when I pass the half way point on fuel, depending on sea state.

 

[Stee6043]

Also keep in mind the larger boat has a lot more hull planing area, and the mechanicals are further forward so they are not as tail heavy as the 340. They actually plane faster, run more level and stay in plane longer at slower speeds, regardless of number of passengers or gear. That eases the load on the engines, so it’s not an apples to apples comparison.
I run around 25mph, at 3300 and have never had a moment I felt I needed more. It’s thirsty, but only boating for four months out of the year you’ll never make up the cost in fuel savings. I say just do it!

This is something I don't think anyone will ever believe! I went from a 99 340 to a 97 400, both gas 7.4's. The overall performance and handling (on plane) of my 400 is worlds better than my 340 ever was. Planes faster, stays on plane slower, less tab required, all of it. I was shocked when I sea trialed the 400.

What I do miss is the slow speed handling of the 340. With 4 blade props that 340 would turn on a dime. You gotta get used to how the gas 400's move when docking. They can be...stubborn.

 

[K-DUB]

This is something I don't think anyone will ever believe! I went from a 99 340 to a 97 400, both gas 7.4's. The overall performance and handling (on plane) of my 400 is worlds better than my 340 ever was. Planes faster, stays on plane slower, less tab required, all of it. I was shocked when I sea trialed the 400.

What I do miss is the slow speed handling of the 340. With 4 blade props that 340 would turn on a dime. You gotta get used to how the gas 400's move when docking. They can be...stubborn.

Definitely. They have a bit of a mind of their own as far as marina manners. Thats where I will tip my hat to the diesel torque.

 

[Firemedic181]

I am considering a 2009 40 Sundancer with just the standard 8.1 370 motors. I would really like to have diesels or at least the 8.1 HO. The boat is really clean and has every option. Would this boat be underpowered?

Previous boat of ours was a 2000 410 Sundancer with the 454’s in her, some said it would be underpowered but I never had any issues with it at all.
Multiple trips that were in the 80 mile range and yes the fuel burn was higher but if it’s the boat you like and want then go for it.
Everyone has something negative to say no matter what you choose but in the end you have to be happy.

 

[Firemedic181]

This is something I don't think anyone will ever believe! I went from a 99 340 to a 97 400, both gas 7.4's. The overall performance and handling (on plane) of my 400 is worlds better than my 340 ever was. Planes faster, stays on plane slower, less tab required, all of it. I was shocked when I sea trialed the 400.

What I do miss is the slow speed handling of the 340. With 4 blade props that 340 would turn on a dime. You gotta get used to how the gas 400's move when docking. They can be...stubborn.

Stubborn is a good word for it,lol
Without that diesel torque I had to use the throttles when docking in those windy days.

So happy with the bigger boat and diesels now.